The prior art has disclosed the practice of using a wide-angle objective with a small focal length (e.g. a focal length of approximately 30 mm) when imaging an object with large angular fields (e.g. with an angular range of greater than 70°). Said wide-angle objective generally comprises a first lens unit with negative refractive power and a second lens unit with positive refractive power. For imaging an object with small angular fields (e.g. with an angular range of less than 25°), use is generally made of a tele-objective with a large focal length (e.g. a focal length of approximately 100 mm).
Furthermore, the prior art has disclosed zoom-objectives, which have a focal-length range extending from a wide-angle range into a tele-range. The focal length of these zoom-objectives can be set. By way of example, U.S. Pat. No. 5,847,875, which is incorporated herein by reference, has disclosed a zoom-lens system. The known zoom-lens system comprises a first lens unit consisting of a lens group with negative refractive power, a second lens unit consisting of a first lens group and a second lens group, wherein both lens groups have positive refractive power, a third lens unit consisting of a lens group with negative refractive power and a fourth lens unit consisting of a lens group with positive refractive power. Furthermore, a stop is arranged between the second lens unit and the third lens unit. All lens groups within the four aforementioned lens units can be displaced along an optical axis in order to adjust the focal length. The position of the first lens group of the second lens unit can additionally be set for focusing purposes. The image plane of the zoom-lens system is not stable during the zoom procedure. Refocusing is necessary for this reason.
JP 08005921 A, which is incorporated herein by reference, has likewise disclosed a zoom-lens system. The zoom-lens system known from this document comprises a first lens unit consisting of a lens group with negative refractive power, a second lens unit consisting of a lens group with positive refractive power, a third lens unit consisting of a lens group with negative refractive power, a fourth lens unit consisting of a lens group with positive refractive power and a fifth lens unit consisting of a lens group with positive refractive power. A stop is arranged between the second lens unit and the third lens unit. In order to adjust the focal length of the zoom-lens system, the lens groups within the second lens unit, the third lens unit and the fourth lens unit are moved. By contrast, the first lens unit and the fifth lens unit have a fixed position when adjusting the focal length. They are not moved when adjusting the focal length.
U.S. Pat. No. 3,682,534, which is incorporated herein by reference, has disclosed a zoom-objective which consists of several lens units. The known zoom-objective comprises a first lens unit consisting of a first lens group with negative refractive power and a second lens group with negative refractive power, a second lens unit consisting of a lens group with positive refractive power, a third lens unit consisting of a lens group with negative refractive power, a fourth lens unit consisting of a lens group with positive refractive power and a fifth lens unit consisting of a lens group with positive refractive power. A stop is arranged between the fourth lens unit and the fifth lens unit. In order to adjust the focal length, the lens groups within the second lens unit, the third lens unit and the fourth lens unit are displaced along the optical axis of the zoom-objective in such a way that the lens group of the second lens unit and the lens group of the fourth lens unit carry out a common, mechanically coupled movement. The first lens unit and the fifth lens unit are fixed when adjusting the focal length. Accordingly, they are not moved when adjusting the focal length. Focusing onto an object to be imaged is achieved by moving the second lens group of the first lens unit.
Accordingly, it would be desirable to specify an optical system (for example an objective for a wide-angle range of greater than 70°), which has an improved design and improved imaging properties. In particular, the image plane should be stabilized.